1. Field of the Invention
This invention relates to a camera for making light measurement by dividing a field of view into plural areas.
2. Description of the Related Art
There have heretofore been proposed various light measuring devices of the type wherein a field of view is divided into plural areas, a luminance value signal is outputted for each area, and the outputted plural luminance value signals are employed to give the photographed image plane correct exposure.
For example, Japanese Utility Model Publication No. Sho 51-9271 proposes a light measuring device in which outputs from plural photoelectric transducers are processed to calculate an arithmetical mean value of maximum and minimum ones thereamong which is used as a light measurement value.
Also, Japanese Patent Publication No. Sho 63-7330 proposes a light measuring device in which a field of view is divided into plural areas, light measurement is effected for a central area and other plural outer areas arranged to surround the central area, the luminance of each area is standardized by using a reference value set between maximum and minimum values of the luminance for those areas, and the field of view is grouped on the basis of the standardized output to calculate a light measurement value by using an output corresponding to the determined group.
In the foregoing prior-art light measuring devices, arrangement of a main object in the photographed image plane has not been properly taken into account. In anticipation of mounting the light measuring device on the camera having an automatic focus detecting device or the like, however, it has also been proposed to take due consideration of the arrangement of a main object in the photographed image plane.
U.S. Pat. No. 4,912,495 proposes a light measuring device in which a main object is assumed to be positioned in a central portion of the image plane, and a field of view is divided into multiple areas comprising at least a plurality of concentric areas about the central portion of the image plane. By assuming the size of the main object based on information of photographing magnification, luminance value signals for the concentric areas are selectively employed so that luminance of the main object may be obtained precisely. This U.S. Patent also proposes to calculate luminance of the background in addition to the luminance of the main object, to thereby change the formula used to calculate a light measurement value by utilizing the luminance difference between the background and the main object. This enables the light measuring device not only to provide correct exposure for the main object, but also to provide correct exposure in match with the photographing situations when the main object is small.
Further, U.S. Pat. No. 4,786,935 proposes a light measuring device in which a main object is assumed to be conveniently located in a central portion of the image plane, and a field of view is divided into at least three areas; i.e., an area at the central portion of the image plane, an intermediate area, and an outermost area. Based on luminance value signals from those plural areas and the differences between the luminance value signals from every two areas adjacent to each other, the approximate size of the main object and the photographing situations are determined simultaneously. This U.S. patent is advantageous in that because the approximate size of the main object is determined by utilizing the difference between the luminance value signals, the light measurement is less dependent on the actual size of the main object than the case of presuming the size of the main object by using the photographing magnification, and correct exposure can be obtained stably.
In the above-cited two prior-art light measuring devices, the main object is assumed to be conveniently located in a central portion of the image plane for the reason that cameras equipped with automatic focus detecting devices have their focus detecting areas generally set in the central portion of the image plane. Meanwhile, there has been recently proposed an automatic focus detecting device which has a plurality of focus detecting areas. In a camera equipped with such an automatic focus detecting device, a main object can be conveniently located in any of the plural focus detecting areas, including any particular one of the areas other than the central portion of the image plane.
In view of the above, Japanese Laid-Open Patent Application No. Hei 1-202720 proposes a light measuring device suitable for such a camera having plural focus detecting areas. As an applied improvement of Japanese Laid-Open Patent Application No. Sho 62-203022 which determines luminance of a main object based on information of photographing magnification, the above patent application discloses a calculation to modify weights for one light measuring area and another light measuring area based on information of one selected focus detecting area and information of the focused condition of the other focus detecting area when the selected area is in an in-focus state, as well as information of photographing magnification and information of the focal length of a photographing lens. The luminance of the main object is thereby measured precisely and outputted as a light measurement value for photographing under natural light.
While a variety of light measuring devices have been proposed which divide a field of view into plural areas and calculate a light measurement value through various arithmetic operations, as explained above, such devices for precisely determining whether flash illumination should be triggered or not have been proposed in a relatively small number. As a device for detecting only a backlighting condition, for example, Japanese Laid-Open Patent Application No. Hei 1-280737 proposes a device for detecting a backlighting condition based on the luminance difference between predetermined areas.
Because of the arrangement of dividing a field of view into plural areas and detecting only a backlighting condition based on luminance difference between the predetermined area, the prior-art device of Japanese Laid-Open Patent Application No. Hei 1-280737 is not suitable for cameras which can change focus detecting points, and has suffered from the problem that a backlighting condition of a main object cannot be precisely detected by using only the luminance difference between predetermined areas. As the device of the above Patent Application cannot detect an object of low luminance at the same time, it is of course desirable to have additional means for detecting an object of the low luminance in determining conditions whether or not to trigger the flash illumination. Another problem is that since a backlighting condition is detected using only the luminance difference, situations of a field of view are not so sufficiently recognized as to permit precise detection of a backlighting condition.